In large print systems multiple system calibrations are performed by sensing the position of printed marks and making adjustments based on the results of these measurements. Often multiple sensor systems are employed to perform each of the calibrations because the required qualities of the sensors, for example, spatial resolution, differ from application to application. Such system according to prior art is schematically shown in FIG. 11.
In a print system for wide receivers it is often necessary to align multiple print elements so they can function as one wide print station to span the width of the receiver. For example, in large inkjet printers, multiple 6″ wide print elements are combined to print on 19″ or 25″ wide paper or other print media imaging e.g. 17″ or 23″ wide (allowing for some overlap between print elements). In another example, multiple LED-exposure devices, each of them 14″ wide, might be combined to cover a receiver width of 28″ in a large electrophographic print system imaging e.g. 27″ wide (allowing for some overlap between print elements). Since the print elements cannot be mounted end to end they are offset from each other in the direction of media travel. To print a straight line of data on the receiver, the printing on each print element must be enabled at different times so that the image is printed in alignment on the receiver. This timing adjustment produces alignment in the direction of media travel.
In large print systems, multiple successive print stations are arranged sequentially to produce the desired output on the receiver by successive printing steps. For a four color print system, four sequential print stations deposit the four process colors cyan, magenta, yellow, and black to create a four-color print. A second set of four print stations might follow to deposit another set of four colors on the reverse side of the receiver. In such a printing system, each color plane on a side of the receiver corresponds to a single image plane. For printing processes producing electrical circuits, the print system might consist of just three print stations to deposit a conductive layer, an insulating layer and a second conductive layer on a flexible print media each of the deposited layers being a separate image plane. Each print station in the print system deposits one image plane according to the design and sequence of the individual print station in the print system. The quality of the printed output is dependent on the accuracy in placing printed dots within each print station composed of various print elements and the accuracy of placing the printed dots of one image plane (printed by one print station) to the printed dots of another image plane (printed by another print station).
For a print stations composed of multiple print elements, there must be a certain amount of overlap between print elements in the cross travel direction to be able to compensate for mechanical tolerances in the assembly. Alignment in the cross travel direction is achieved by selecting the smallest addressable picture element (pixel or printed dot) on which one print element stops printing and the next print element starts printing. A method to align the print elements is to print marks from each print element, measure the marks, and adjust the print timing (e.g. with line delays) and overlap pixel for optimal printing. The process to select the correct time delay between print elements (in direction of the print process) and the selection of the printed dot printed using one print element and the adjacent printed dot printed using the adjacent print element (in direction perpendicular to the print process) is referred to as stitching. It is the objective of this stitching process to print a line of a single printed dot (pixel) wide across the entire width of the receiver without discernible artifacts indicative of missing dots in the printed line or misplaced dots not in-line with a straight line.
For this first application, a common method to do this is to use high-resolution digital cameras to measure marks from each print element and make the adjustments. Similarly, for even larger printing systems multiple print stations are combined side-to-side to form a wide-format print system covering a total printing width of multiple print stations wide.
In a high quality multi-color print system the individual color planes are generated by separate print stations. The individual color planes should be printed directly on top of each other. Even though mechanical alignment of the print stations might be perfect with respect to a mechanical datum, conveyance of the flexible web-based receiver (paper or plastic) along the print stations in the printer will generally not enable the individual colors to be printed on top of each other due to variation in receiver motion past the print stations and to dimensional changes of the web (stretching, shrinkage) along the conveyance path. Any pixel placement error is called misregistration and is unacceptable.
For a second application, a common method to maintain good registration is to measure the positions of the colors regularly and adjust the positions of the colors during the print process. The first and second applications require a high-resolution imaging device to measure and evaluate the captured image for necessary corrections for good registration and artifact free transition between print elements. The latter is also referred to as stitching of images without artifacts. Test patterns overlapping the transition from one print element to the adjacent to assess the stitch quality can either be printed in a special calibration routine or placed in the area between output images during normal print production. In both cases, the controller sequences the generation of test patterns and, in concert, sequences the capture and evaluation of test patterns to yield the required correction for color placement and exposure timing and pixel overlap.
A third application includes a group of functions intended to assess the quality of the printing process. This third application commonly includes the detection of defects such as streaks, or other unintended density non-uniformity within a page, or missing lines of data and the visualization of completed images as they are printed.
A fourth application includes the evaluation of printed density in the output. Either predetermined areas (color patches) generated in between output images or user defined areas of the output images are processed and compared with either predetermined target values or customer defined target density values. For the third and fourth application, a low-resolution imaging device is sufficient to capture streaks, missing lines or evaluate the color of defined test areas.
Current implementations use multiple sensor systems for these functions. For example, multiple high-resolution cameras with small fields of view can be used for the first two applications while a line array with a wide field of view can be used for the third and fourth application. Such arrangement of multiple sensor systems according to prior art is schematically shown in FIG. 11. Because of bandwidth limitations in the data acquisition electronics, it is not practical to acquire the full image width at high-resolution. The processing of large amount of data at high speed becomes very expensive.